Heretofore, for example, in an image forming apparatus having a photosensitive drum and performing an image formation by an electrophotography system, it is necessary to properly charge, that is, typically uniformly charge a potential of the photosensitive drum under any circumstance in order to constantly obtain a stabilized image quality. Hence, the charged potential of the photosensitive drum is measured by using an electric potential measuring device (hereinafter, also referred to as “potential sensor”), and by utilizing the measurement result, a feed back control is performed so as to uniformly keep the potential of the photosensitive drum.
As for a conventional potential sensor, there is available a non-contact type potential sensor. Here, a system called as a mechanically modulated alternating electric field induction type is often used. According to this system, the potential of the surface of a measuring object is a function of the magnitude of the current taken out from a detection electrode contained in the potential sensor, given by the following formula:i=dQ/dt=d(CV)/dt  (1)
Here, Q denotes a charge to appear on the detection electrode, C a coupling capacitance between the detection electrode and the measuring object, and V an electric potential of the surface of the measuring object, and this capacity C is given by the following formula:C=AS/x  (2)
Here, A denotes a proportionality constant, S a detection electrode area, and x a distance between the detection electrode and the measuring object.
Although a potential V of the surface of the measuring object is measured by using these relationships, in order to accurately measure the charge Q to appear on the detection electrode, it has been so far understood that the magnitude of the capacitance C between the detection electrode and the measuring object is better to be periodically modulated. As for the modulating method of this capacitance C, there are available two types of the method: a vibration capacitance type in which a distance x between the detection electrode and the measuring object is changed, and a chopper type in which an effective detection electrode area S is changed. As for the chopper type, the following method is known.
A first method is to effectively modulate the area S of the detection electrode. A fork-shaped metal shutter is inserted between the measuring object and the detection electrode, and the shutter is periodically moved in a parallel direction with the surface of the measuring object, so that a shielding extent of the electric lines of force from the measuring object reaching on the detection electrode is changed and the area of the detection electrode is effectively changed, thereby realizing the modulation of an electrostatic capacitance C between the measuring object and the detection electrode (see U.S. Pat. No. 4,720,682).
The second method is to dispose a metal shield member having an aperture in a position opposite to the measuring object and provide the detection electrode in a top end of the fork-shaped vibration element so that the position of the detection electrode is changed in parallel directly below the aperture, thereby modulating the number of electric lines of force reaching the detection electrode and modulating the electrostatic capacitance C (see U.S. Pat. No. 3,852,667).
On the other hand, in order to downsize the electrophotographic image forming apparatus, it is necessary to effect reduction in the diameter of the photosensitive drum and increase in the packaging density of parts neighboring the drum, and at the same time, downsizings as well as a thin down of the potential sensor are required. However, in the above-described sensor of the current mechanically modulated alternating electric field induction type, the inner volume of a sensor structure is occupied by assembling parts such as a drive mechanism and the like for vibrating the fork-shaped shutter or the fork-shaped vibration element. Hence, for the downsizing of the potential sensor, it is essential to downsize these drive mechanisms.
In recent year, experiments have been reported in which by utilizing a semiconductor processing technology called as Micro Electro Mechanical System (MEMS), a fine mechanical structure is formed on a semiconductor substrate. The mechanically modulated alternating electric field induction type potential sensor using such technology has been also reported. As a typical example, there is a case in which a shutter structure having the fine opening portion prepared by the semiconductor processing technology is vibrated directly above the detection electrode, thereby attempting to measure the electric potential of the measuring object (see U.S. Pat. No. 6,177,800).